IN THE GARDEN; With Warmer Weather, Different Decisions to Make

By ANNE RAVER

Published: December 21, 2006

IT'S not that I don't like 60-degree days and eating fresh spinach right out of my garden in December. But the extended growing season is one of the signs of global warming. It goes hand in hand with polar bears dying in the Arctic as the sea ice shrinks.

For the gardener, there are benefits and there are drawbacks.

In central Maryland, warmer winters allow me to grow Southern magnolias and apricot trees, but more insects are wintering over, and weeds, like poison ivy and ragweed, seem far more aggressive. It didn't surprise me to see that my garden has jumped a zone, from 7 to 8, according to a hardiness zone map based on lowest winter temperatures in the past 15 years, just published by the National Arbor Day Foundation (arborday.org/media/zones.cfm).

The Agriculture Department's Research Service is also revising its hardiness zone map, based on temperatures over the past 30 years, so stay tuned.

Cameron P. Wake, a research associate professor at the Climate Change Research Center at the University of New Hampshire, said that winter temperatures in the Northeast have increased an average of 4.3 degrees over the last 30 years.

That may not seem like very much, Mr. Wake said. But picture Boston experiencing a winter climate more like Philadelphia's; or Philadelphia having winters more typical of Washington.

All kinds of plants are changing their patterns. According to research by David W. Wolfe, a professor of horticulture at Cornell University, lilacs are blooming an average of four days earlier than they were in the 1960s; apple trees and grapevines are blooming about eight days earlier.

That can have a downside. Earlier spring warming does not rule out sudden drops in temperature. If the trees are blooming earlier and there is a cold spell, there is a risk of frost damage to flowers and to developing fruit.

Apple trees need a winter chilling period of at least 30 consecutive days with temperatures below 40 degrees, Mr. Wolfe said. And winter thaws that interrupt that chilling period inhibit fruit production.

As the warming trend continues, gardeners and farmers are changing the varieties of vegetables and fruits that they grow.

Prudence Wickham Heston, a partner in Wickham's Fruit Farm in Cutchogue, N.Y., described crops that once would not have survived on the farm. Her family is growing peach and apricot trees down near the Peconic Bay, which holds warmth in the fall and stays cool in the spring, producing ideal conditions for these crops. They have also planted kiwi vines, which would have been unheard of in Grandfather John Wickham's day, when people used to drive across the frozen bay to Southampton.

But Tom Wickham, Ms. Heston's uncle, worries that the mild winters will bring more insects and disease.

''Apple scab will over-winter in dead leaves and twigs, but a really cold winter, of about zero degrees, will suppress it,'' Mr. Wickham said. ''We used to have winters well below zero, but we haven't had that for a long time.''

Warmer temperatures have also benefited Long Island's grape growers, who are starting to plant syrahs, which need more heat to develop flavor, said Louisa Thomas Hargrave, a former vintner who is the director of the Center for Wine, Food and Culture at Stony Brook University. Grape growers worry, however, about another aspect of global warming: earlier and more intense hurricanes that could wipe out a crop.

''When Gloria hit in 1985, it came loaded with spray from the sea, and all that salt was deposited on the leaves,'' said Charles Massoud, who with his wife, Ursula, owns Paumanok, a 75-acre vineyard in Aquebogue, N.Y. Luckily, the Massouds were able to harvest their grapes the day before the storm struck, on Sept. 27.

As for the weeds in our lives, they are growing bigger and faster, and producing more seeds, as temperatures rise.

''Japanese honeysuckle and English ivy seem to be responding very well to CO2 and warmer temperatures,'' said Lewis Ziska, a weed scientist based at the Agriculture Department's office in Beltsville, Md.

And they are more resistant to herbicides, he said. Canada thistle, for example, grown in a chamber of elevated carbon dioxide, was not killed until sprayed with three times the regular dose of herbicide.

And each ragweed plant ''can produce 10 times the amount of pollen, just because of climate change and CO2,'' he said, making the pollen counts much higher.

So what does all this mean for the gardener?

Obviously, we can have some fun pushing the envelope. But on the more serious side, we need to be setting rain barrels under our roofs to harvest the increasing number of torrential downpours.

''More of our water is getting squeezed into high-precipitation events,'' Mr. Wolfe said, meaning a rainfall of more than two inches within 48 hours. ''That increases the danger of drought, because there is less water in between.''

So get out the drip lines and connect them to those barrels, to water during hot, dry spells.

There are all kinds of tricks of the trade, like planting spinach and lettuce in the shade, and choosing more heat-tolerant varieties, to offset higher temperatures.

Composting is ever more important, not only to return organic material to the soil, but to keep it from landfills and incinerators, which increase the methane and CO2 pouring into the atmosphere.

In fact, every time we till the soil, or dig it up, we are releasing more CO2. Microbes need air to break down carbon, and by tilling, or digging, they are getting just that, and the result is CO2.

So avoid the urge to dig and till. Make those raised beds so friable -- with plenty of compost -- that you can just make holes and drop the plants into them. I can make a row for seeds with my fingertip.

Planting cover crops like winter rye, buckwheat and clover can also slow the release of carbon, by sequestering it in plant roots. Bare soil can be covered with a blanket of chopped leaves. Limiting the use of chemical fertilizer, which is produced by the burning of fossil fuel, is helpful. And remember, the use of too much chemical fertilizer overloads the soil with nitrogen, which gives off nitrous oxide, another greenhouse gas.

If a lawn is too big for a people-powered reel mower, it can be shrunk down by planting ground covers and trees, which will take more carbon dioxide out of the air.

The same principle applies to roofs and terraces: more plants absorb more carbon dioxide.

And why not get rid of the leaf blower and the gasoline-powered lawn mower? According to the California Air Resources Board, operating a lawn mower for an hour creates about as much pollution as driving a car from Washington to New York City.

Of course, gardens alone can't save the polar bears, or all the other species that could vanish. Only a radical reduction in the burning of fossil fuels can do that.

Photos: ENDLESS SUMMER -- Tom Wickham planted kiwis on Long Island, left. But an increase in hurricanes could hurt vineyards like Paumanok, above and below. (Photographs by Gordon M. Grant for The New York Times)